Semiconductor devices having a double-layer interconnection structure

ABSTRACT

A semiconductor device having a double-layer interconnection with contact portions between first and second metal films covered with at least a silicon nitride film is provided wherein an electromigration characteristic at the contact portions is improved. The improvement is achieved by defining a value obtained by multiplying a thickness of the silicon nitride film by a stress of the nitride film formed at the contact portions is not larger than 2/5 of a value obtained by multiplying a thickness of the silicon nitride film by a stress of the nitride film formed at non-contact portions. By this, the stress exerted on the second metal film is reduced to improve the electromigration life at the contact portions by about one order of magnitude. The first and second metal films are made of Al or Al-based alloys.

BACKGROUND OF THE INVENTION

1. Field of The Invention

This invention relates to electronic devices and more particularly, tosemiconductor devices having a super LSI multilayer interconnectionstructure.

2. Description of The Prior Art

As is known in the art, the high degree of integration of super largescale integration circuits invariably requires multilayer or multilevelinterconnections where the interconnection structure has two or morelayers built up therein. Semiconductor devices having such aninterconnection structure include a protective layer for preventingmoisture and/or impurities from entering. This protective layer is tocover the multilayer interconnection therewith and has usually astructure which includes a phosphosilicate glass layer (P-containingSiO₂) and a silicon nitride film formed by plasma-enhanced CVD. Atypical example of a two-layer wiring structure is illustrated withreference to FIG. 9. In the figure, there is generally shown asemiconductor device S which includes a substrate 20 having a transistorregion (not shown) covered with an insulating film 21 having an opening21a. A first metal film 22 serving as a first wiring is formed on thesubstrate 20 as shown. Moreover, an insulating film 23 having an opening23a is formed over the first metal film 22 and a second metal film 24serving as a second wiring is provided on the insulating film 23. Thesecond metal film 24 is in contact, as C, with the first metal film 22through the opening 23a. Finally, a protective layer 25 having adouble-layered structure of a phosphosilicate glass layer and a siliconnitride layer (both not shown) is formed to cover the entire surface ofthe semiconductor device, thereby completing a two-layerinterconnection. This interconnection structure is fundamentallycomposed of three lines including a line of the first wiring 22, a lineof the second wiring 24 and the contact portion C where the first wiring22 and the second wiring 24 are electrically contacted.

The degree of integration of large scale integrated circuits becomeshigher with a finer size of the metal films or wirings. When an electriccurrent is applied to such a circuit, there will arise the problem thatthe breakage of the fine wiring takes place. This is calledelectromigration (which may be hereinafter referred to often as EM)failure. This EM failure takes place due to the fact that Al atomsordinarily used as the metal film are moved along the passage directionof electrons generated by application of the current and a portion ofthe film where the movement is violent is broken. To avoid this, it hasbeen proposed that elements other than Al, e.g. Cu, Ti and the like, areadded to the Al film so that the number of vacancies present in Al grainboundaries are decreased to reduce the movement of Al atoms (P. B. Gate,Solid State Technology, Vol. 3, pp. 113-120, (1983)).

Another problem involved in the fine wiring of the super large scaleintegrated circuits is a phenomenon where when kept merely at hightemperatures, the wiring is broken. This is called stress migration(which may be often referred to as SM). With regard to the SMphenomenon, it has been reported that the breakage frequently takesplace when the circuit is maintained at a temperature of approximately150° C. This will be overcome to an extent by addition of Cu or the likeelement (J. Kelma et al, The 22nd Annual Proceeding InternationalReliability Physics Symposium, pp. 1-5 (1984)).

The above two reports have been made to solve the problems on thesingle-layer wiring line. With LSI circuits including a double-layer ormultilayer interconnection structure, the problems involved in the EMand SM phenomena at contact portions between the interconnected wiringshave to be solved, like the single-layer wiring line.

Few reports have been made on the EM and SM phenomena at the contactportion. It is empirically assumed that the EM and SM characteristicswill be improved by tapering the insulating film at the contact portionto improve the aluminium coverage of the second wiring. This taperedtechnique has now been in use. However, it has been reported that thistechnique has the following disadvantage (H. Tomioka et al, The 27thAnnual Proceeding International Reliablility Physics Symposium, p. 57(1989)). The second aluminium film is usually formed by sputtering atrelatively high sputtering power. In this condition, when the contactportion is tapered, the substrate made generally of SiO₂ is apt to besputtered at the tapered contact portion as well. This results in SiO₂formed between the first and second aluminium films, thereby causing theEM characteristics to be degraded. Thus, care should be paid to theformation of SiO₂ at the contact interface.

The status of the double-layer wiring arrangements can be summarized inthe following table.

                  TABLE                                                           ______________________________________                                                                    Possibility for                                                               Application                                       Prior     Measure for Solving                                                                             to Semiconductor                                  Art       EM Problem        Device                                            ______________________________________                                        EM Phenomenon                                                                 first several *improved by addition of                                                                        possible to use                               metal reports impurity elements in 1.2 μm line                             film          *improved by increas-                                                                           width                                                       ing grains of Al                                                second                                                                              several *improved by addition of                                                                        possible to use                               metal reports impurity elements in 1.2 μm line                             film          *improved by increasing                                                                         width                                                       grains of Al                                                    contact                                                                             few     *because of poor coverage                                                                       difficult to use                              portion                                                                             reports of the second metal film                                                                        in a size of                                                at the contact portion,                                                                         1.2 μm square                                            the portion is tapered but                                                    with a problem that SiO2 is                                                   formed at the interface                                                       between the first and                                                         second metal film                                               SM Phenomenon                                                                 first several improved by addition of                                                                         possible to use                               metal reports impurity elements in 1.2 μm line                             film                            width                                         second                                                                              several improved by addition of                                                                         possible to use                               metal reports impurity elements in 1.2 μm line                             film                            width                                         contact                                                                             few     *because of poor coverage                                                                       not known                                     portion                                                                             reports of the second metal film,                                                     the portion is tapered, which                                                 is now under study                                              ______________________________________                                    

As will be apparent from the above table, although EM and SMcharacteristics of the first and second metal films and the problemsinvolved in the contact portion have been individually reported, we havenot found any attempt to match the respective films and the contactportion when the double-layer wiring structure is applied tosemiconductor devices.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a semiconductordevice having a double-layer interconnection structure with improvedelectromigration and stress migration characteristics wherein therespective metal films and a contact portion of the films are so matchedas to improve the electromigration characteristics, particularly, of thecontact portion.

It is another object of the invention to provide a semiconductor devicehaving a double-layer interconnection structure wherein a metal filmcovered with a protective layer and formed at a contact portion of firstand second metal films is reduced in stress from the protective layer.

It is a further object of the invention to provide a semiconductordevice having a double-layer interconnection structure whoseelectromigration life is improved by one order of magnitude over knowninterconnection structure.

The above objects can be achieved, according to the invention, by asemiconductor device of the type which comprises a first metal filmformed on a semiconductor substrate, an insulating film having openingsand formed on the first metal film, a second metal film provided inelectric connection with the first metal film through the openings ofthe insulating film, and a protective layer made of a phosphosilicateglass film and a silicon nitride film formed on the second metal film inthis order. In the semiconductor device, the improvement resides in thata value obtained by multiplying a thickness of the silicon nitride filmformed on the second metal film layer at the openings by a stress of thesilicon nitride film is not larger than 2/5 of a value of a thickness ofthe silicon nitride film formed on the second metal film on portionsother than the openings by a stress of the silicon nitride film on theportions.

The openings may be tapered or not. If tapered, an angle of the taper ispreferably in the range of not less than 75°.

The first and second metal films may be made of Al alone or Al alloyscontaining Si, Cu, Ti and/or Pd in amounts of up to 5 wt% in total.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a to 1c are, respectively, a semiconductor device according to anembodiment of the invention wherein FIG. 1a is a schematic sectionalview of a contact chain portion used to measure EM life of a multilayerinterconnection, FIG. 1b is a schematic sectional view of a first metalfilm or first wiring, and FIG. 1c is a schematic sectional view of asecond metal film or second wiring;

FIG. 2 is a schematic sectional view of a contact portion between thefirst metal film and the second metal film according to the invention;

FIGS. 3a to 3c are, respectively, graphical representations wherein FIG.3a is a graph showing the relation between the coverage rate ofaluminium and the angle of taper at a contact portion, FIG. 3b is agraph showing the relation between the thickness of a SiN protectivefilm and the angle of taper at the contact portion, and FIG. 3c is agraph showing the relation between the average failure time and theangle of taper at the contact portion;

FIG. 4 is a graphical representation of the relation between thethickness of a SiN protective film and the average failure time;

FIG. 5 is a graphical representation of the relation between the totalstress and the thickness of a SiN protective film;

FIG. 6 is a graphical representation of the relation between the averagefailure time and the total stress;

FIG. 7a is a graphical representation of the relation between thecumulative failure rate and the test time for semiconductor devices ofthe invention and for comparison,

FIG. 7b is a graphical representation of the relation between the wiringlife and the test time;

FIG. 8 is a graphical representation of the relation between the percentdefective and the retention time at 150° C. for a stress migrationcharacteristic of a contact chain;

FIG. 9 is an illustrative sectional view of a double-layerinterconnection in a known semiconductor device;

FIG. 10 is a schematic plan view of a contact portion of a double-layerinterconnection of a semiconductor device according to the invention;

FIG. 11 is a graphical representation of the relation between thecumulative failure rate and the test time for illustrating anelectromigration characteristic of a first metal film, a second metalfilm and a contact chain between the first and second metal films,respectively; and

FIG. 12 is a graphical representation of the relation between thecontact chain resistance after thermal treatment and the contact chainresistance prior to the thermal treatment.

DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION

Initially, the reason why we attempted to improve the EM characteristicat contact portions between first and second metal films is describedwith reference to FIGS. 1 and 10 to 12 wherein like reference numeralsindicate like parts.

The sizes of the first and second metal films and the contact portionsusually employed in semiconductor devices are first described. Forinstance, when the width of the first metal film is set at a minimumlevel for microprocessors where a great load is applied to the metalfilm, e.g. 1.2 μm, the width of the second metal film is determined at aminimum level of 1.2 μm. The contact portion is determined to have amaximum size at a portion where the first and second metal films arecrossed. The area of the contact portion becomes 1.2 micrometer square.This is particularly shown in FIG. 10. In the figure, the alignmentmargin extending from first and second metal films 12, 14 and providedaround contact portions C is an allowance in anticipation of analignment shear in lithography. Thus, the sizes of the first and secondmetal films 12, 14 and the contact portion C used to make a double-layerinterconnection structure are determined such that the first and secondmetal films 12, 14 have the same width and the contact portion C is aquadrilateral with one side corresponding to the width.

The EM characteristic of the double-layer interconnection structure asshown in FIG. 10 is measured. The double-layer interconnection structureconsists of the first and second metal films and the contact portion andthe sections of these portions are shown particularly in FIGS. 1a to 1c.For the measurement, an aluminium alloy containing 1% of Si and 0.5% ofCu is used as the first and second metal films 12, 14. 400 contactportions C of the first and second metal films 12, 14 with 1.2micrometer square are connected in series (hereinafter referred to as acontact chain) as shown in FIG. 1a. The device of FIG. 1a will bedescribed in detail hereinafter.

Since a step coverage of the aluminium alloy by sputtering is relativelypoor, each contact portion is tapered for convenience' sake at about 55°in a series of experiments made by us only for confirmation.

In addition to the double-layer interconnection shown in FIG. 1a, awiring structure including the first metal film 12 having a width of 1.2μm and a thickness of 0.8 μm is shown in FIG. 1b. Likewise, a wiringstructure including the second metal film 14 having a width of 1.2 μmand a thickness of 1.0 μm is shown in FIG. 1c. The lives of therespective structures are determined by EM measurement.

The EM measurement is made in the following manner.

After determination of the sectional areas of the first and second metalfilms and the contact portions, the respective structures are kept at150° C. and applied with an electric current with a current density of2×10⁶ A/cm² to determine a time before it fails. This is usually calledEM measurement. It will be noted that the EM measurement is anacceleration test wherein a life test corresponding to 10 years iseffected in about 100 hours. The upper limit of actual use conditions ofLSI circuits include those conditions of a temperature of 80° C. and aDC current density of 1×10⁵ A/cm² whereupon a pulse current is passedthrough a 1.2 μm square contact portion. The life of the interconnectionstructure is estimated by keeping a sample at 150° C. and applying a DCcurrent with a current density of 2×10⁶ A/cm². With the contact portion,when the time before it fails is over 12 hours, its actual service lifecorresponds to over 10 years. With the first and second films, the timeover 50 hours corresponds to an actual service life of over 10 years.

The results are shown in FIG. 11. As will be apparent from the figure,the contact chain is shorter in the time by one order of magnitude orover than the other parts.

It has been reported that the EM characteristic at the contact chain isdegraded due to the presence of a thin SiO₂ layer at the contact portionbetween the first and second metal films. More particularly, when thesecond metal film is deposited by sputtering, SiO₂ at the taperedportion is sputtered by means of Ar ions and re-deposited on the firstmetal film exposed at the opening. To avoid this, the sputtering of thealuminium alloy for the second film is performed at a small power levelso that SiO₂ is not sputtered.

The absence of SiO₂ at the interface has been confirmed as follows.

If a thin SiO₂ film is present at the interface, the contact chainresistance after fabrication of the double-layer interconnectionstructure scatters greatly and its value is greatly varied when thestructure is subjected to thermal treatment at about 400° C. A chain of10000 contact portions with a taper angle of 55° is subjected to thermaltreatment at 450° C. for 150 minutes to measure a resistance of thecontact chain. In this measurement, the contact size is 0.8 μm, 1.0 μm,and 1.2 μm square. 108 samples for each contact size are used for themeasurement. The variation of the resistance prior to and after thethermal treatment is shown in FIG. 12.

As will be seen from the figure, the resistance is scattered to anextent for the contact size of 0.8 μm but a significant variation in theresistance is not found prior to and after the thermal treatment for theother sizes. This means that SiO₂ is not present at the contact portion.For further confirmation from a physical aspect, the section of the 1.2μm contact portion has been subjected to observation through atransmission electron microscope, with the result that no SiO₂ is found.The aluminium has been found to be alloyed at the contact portion.

In view of the above results, we made further studies to solve theproblem that the EM characteristic at the contact portions between thefirst and second metal films is poor irrespective of the absence of SiO₂at the interface and thus to provide a semiconductor device with animproved EM characteristic at the contact portions of the double-layerinterconnection.

Reference is now made to the accompanying drawings and particularly, toFIGS. 1a to 1c.

In FIG. 1a, there is shown a semiconductor device S which includes asemiconductor substrate 10 having a SiO₂ film 11 thereon, a first metalfilm or wiring 12 formed on the SiO₂ film 11. An insulating film 13having openings 13a is formed on the first metal film 12, on which asecond metal film or wiring 14 is formed as connected with the firstmetal film 12 through each opening 13a at a contact portion C with anarea of about 1.2 micrometer square. Further, a protective film 15consisting of a phosphosilicate glass film 15a and an SiN film 15b isformed over the entire surface. The semiconductor substrate 10 has asemiconductor region (not shown) and the first and second metal films12, 14 are made of aluminium containing 1% of Ti and 0.5% of Cu for thefollowing tests although not limited to the Al alloy alone.

FIG. 1b is a schematic view of a single-layer structure including thefirst metal film 12. For instance, the first metal film 12 is 1.2 μm inwidth and 0.8 μm in thickness. Likewise, FIG. 1c is a schematic view ofa single-layer structure including the second metal film 14 having, forexample, a width of 1.2 μm and a thickness of 1.0 μm.

In the present invention, the EM characteristic at the contact portion Cas shown in FIG. 1a is improved. In order to clarify the influence ofthe SiN film 15b, a taper or inclination angle at the contact portionsas shown in FIG. 2 is changed. Then, the average failure time of theresulting semiconductor device is checked in relation to the coveragerate of the second aluminium layer and the thickness of the SiN film.

The coverage rate of the aluminium layer and the thickness of the SiNlayer are determined through observation of a section of the contactportion with a scanning-type electron microscope. Further, samples withdifferent coverage rates of the Al layer are provided wherein 500contact portions are connected in series in each sample. The same levelof electric current is applied to all the samples in such a way that thecurrent density is set at 2×10⁶ A/cm² at a minimum aluminium sectionalarea of the contact portion of each sample with a taper angle of 85° tomeasure an EM value, thereby determining an average failure time. Itwill be noted that the average failure time is one at which a cumulativefailure reaches 50% when the EM value is measured as shown in FIG. 11.The results of the above measurement are shown in FIGS. 3a to 3c.

As will be seen from FIG. 3a, a higher taper angle, θ, at the contactportion results in a lower coverage rate of the aluminium layer.Similarly, FIG. 3b reveals that a higher angle results in a lowerthickness of the SiN film. In contrast, the average failure timeincreases with an increase of the angle, θ, as is seen from FIG. 3c. Inorder to obtain the EM life of the contact chain at the same level asthe EM life of the second aluminium film, the taper angle at the contactportion should preferably be in the range of not less than 75°, morepreferably not less than 80°. It will be noted that in the practice ofthe invention, it is not necessarily intended to define the taper angleat the contact portion, but physical factors which contribute to limitthe EM life of the contact chain are important. In view of the coveragerate of the aluminium layer, the results of FIG. 3 are contrary to thegenerally accepted assumption that an increase of the current densityleads to degradation of the EM characteristic. Accordingly, theinfluence of the SiN film has been investigated in the following manner.

There are fabricated double-layer interconnection structures of the typeas shown in FIG. 2 where any SiN film is not formed in one case, SiNfilms are formed with different thicknesses at the same stress level,and a SiN film is formed whose quantity of stress is reduced to 1/2 ofthe above-mentioned SiN films. In all cases, the taper angle is set at85°. The EM characteristic of each structure is measured to determine anaverage failure time. The results are shown in FIG. 4. In the figure,the mark "◯" is for the SiN films with the same level of stress. In thiscase, it will be found that the average failure time increases with adecrease of the SiN film thickness. In the absence of the SiN film, theaverage failure time is over 100 hours and is thus significantlyimproved with respect to the EM life. This means that the EM life of thecontact chain has close relation with the SiN film. The mark " " in thefigure is for the SiN film whose stress is half the stress of the SiNfilms indicated by the mark "◯". From the results of FIG. 4, it will beseen that the EM characteristic at the contact portion is dependent notonly on the thickness of the SiN film, but also on the stress of the SiNfilm. Thus, the EM life of the double-layer interconnection structure atthe contact portion is greatly influenced by the thickness and thestress of the protective SiN layer. A smaller thickness results in abetter EM characteristic and a smaller stress results in a better EMcharacteristic. The EM characteristic at the contact portion dependssimilarly on the thickness and the stress of the SiN film. This isbecause a degree of deformation of aluminium such as warpage,compression or the like is expressed by a value obtained by multiplyingthe thickness of SiN film by the stress (dynes/cm²). This value will behereinafter referred to as total stress.

In order to numerically express the above relationship, the stress ismeasured for different thickness of the SiN films by the use of theAUTOSORT MARK II 150 instrument, available from GCA Corporation. Thestress of the SiN film at the contact portion is measured by depositingon a Si substrate by plasma-enhanced CVD a SiN film with the samethickness as the SiN film thickness at the contact portion determined bythe SEM technique as in FIG. 3b. It will be noted that although theprotective layer includes the PSG film, the stress of the PSG film isapproximately 108 dynes/cm² and is negligible. The relation between theSiN film thickness and the stress is shown in FIG. 5. The film qualityis changed by changing the deposition conditions of the plasma-enhancedCVD. In the figure, the mark "◯" indicates a 50 nm thick SiN film havinga stress of 3×10⁹ dynes/cm² and the mark " " indicates a 50 nm thick SiNfilm having a stress of 5×10⁹ dynes/cm². These films are subject tomeasurement of the EM characteristic.

The relation between the measured total stress and the average failuretime obtained from the EM value is shown in FIG. 6. As will be apparentfrom the figure, the life of the Al wiring at the contact portion isinversely proportional to the total stress. In order to improve the EMlife by one order of magnitude for the reason set out hereinlater usingthe protective layer having such a stress as mentioned above,consideration should be made to scatterings in measurement of thestress, the SiN film thickness and the life at the contact portion.

This is more particularly explained with regard to FIG. 6. The SiN filmthickness deposited on the second Al film is 500 nm and the total stressat the second Al film is approximately 1.5×10⁵ dynes/cm. Therelationship between the total stress at the contact portion and theaverage failure time may be represented by the solid line in the figure.However, when the scattering of the measurement is taken into account,the relationship will be expressed by the broken line. In addition, theaverage failure time or the EM life of the second Al wiring is in therange of 10 to 20 hours as shown in the figure. In order to prolong theEM life at the contact chain substantially at the same level as that ofthe second wiring or to improve the life by one order of magnitude overthat of a prior art counterpart, the total stress has to be not largerthan approximately 6×10⁴ dynes/cm at the contact portion. This valuecorresponds to about 2/5 of the total stress of the SiN film formed onthe second wiring at portions other than the contact portion. Moreparticularly, the total stress of the SiN film formed on the secondwiring in the opening should be not larger than 2/5 of the total stressof the SiN film formed on the second wiring at portions other than theopening.

The EM characteristic of a prior art double-layer interconnectionstructure having a contact portion with an area of 1.2 micrometer squareand a double-layer interconnection structure of the invention having thesame area of the contact portion as mentioned above where the totalstress is not larger than 2/5 of the total stress of the protective filmon a non-contact area of the Al film or wiring is shown in FIG. 7a. Fromthis, it will be seen that the EM characteristic of the invention isimproved by not less than one order of magnitude over that of the priorart.

The reason why the EM life has to be improved by approximately one orderof magnitude is ascribed to the guaranteed life of semiconductorproducts. The life of LSI semiconductor products is generally determinedby the current density and the use temperature, under which it isguaranteed over approximately ten years. The measurement of the EMcharacteristic is made under accelerating conditions. To this end, theEM measuring conditions in this test include a temperature of 150° C.and current density of 2×10⁶ A/cm². The actual use conditions include atemperature of 80° C. and a current density of 1×10⁵ A/cm² forsubstantially all types of products except for specific applications.The cumulative failure of about 0.3% in FIG. 7a corresponds to 2.83 testhours, which in turn corresponds to a life of the metal film wirings often years under actual use conditions. This relationship is shown inFIG. 7b, revealing that the guaranteed life of ten years can be achievedaccording to the invention.

In the foregoing, the aluminium film or wiring containing Si and Cu hasbeen described. Pure Al with or without Si, Ti and/or Pd may be likewiseused as the film with similar EM characteristic-improving effect.

Then, a stress migration characteristic which is another factor fordetermining the film life is described. 400 contact chains formedthereover with a protective layer made of a 300 nm thick PSG film and a500 nm thick SiN film are connected in series and have taper angle of55° for comparison and 85° used in the present invention. The sample ismaintained at 150° C. to determined a percent defective of film breakagein relation to the time. The results are shown in FIG. 8. In the figure,the mark "Δ" indicates a prior art case where the contact size is 1.0micrometer square, the mark " " indicates a prior art case where thecontact size is 1.2 micrometer square, the mark "□" indicates a case ofthe invention where the contact size is 1.0 micrometer square, and themark " " indicates a case of the invention where the contact size is 1.2micrometer square. No percent defective takes place up to a time of 1000hours in both the prior art and the invention. At 2100 hours for the 1.0micrometer square contact portion, percent defective takes place in boththe prior art and the invention. However, any problem will not beinvolved in the semiconductor products when the time of 1000 hours isensured. Since the breakage failure takes place at 2100 hours both inthe prior art and the invention with a difference therebetween, the SiNfilm at the contact portion is not considered to significantly influencethe stress migration.

As will be apparent from the foregoing, the stress of the SiN film andthe thickness of the SiN film at the contact portion are properlycontrolled so that the value obtained by multifying the thickness of thesilicon nitride formed on the second metal film at the contact portionis not larger than 2/5 of the value obtained by multifying the thicknessof the silicon nitride formed on the second metal film at thenon-contact portion by the stress of the silicon nitride at thenon-contact portion. By this, the stress exterted on the second metalfilm can be suppressed to a low level. The electromigration life of thecontact portion can be improved by about one order of magnitude. The SiNfilm is usually formed in a thickness of from 300 to 1500 nm on portionsof the second metal film other than in openings. In openings, thethickness is in the range of from 120 to 600 nm.

What is claimed is:
 1. In a semiconductor device of the type whichcomprises a first metal film formed on a semiconductor substrate, aninsulating film having openings and formed on the first metal film, asecond metal film provided in electric contact with the first metal filmthrough the openings of the insulating film, and a protective layer madeof a phosphosilicate glass film and silicon nitride film formed on thesecond metal film, the improvement in that a value obtained bymultiplying a thickness of the silicon nitride film formed on the secondmetal film layer at the openings by a stress of the film is not largerthan 2/5 of a value of a thickness of the silicon nitride film formed onthe second metal film on portions other than the openings by a stress ofthe silicon nitride film on the portions.
 2. The semiconductor deviceaccording to claim 1, wherein the first-mentioned value is not less than6×10⁴ dynes/cm.
 3. The semiconductor device according to claim 1,wherein the openings are each tapered.
 4. The semiconductor deviceaccording to claim 1, wherein the first and second metal films are eachmade of a metal selected from the group consisting of Al and Al alloyscontaining at least one alloying element selected from Si, Ti, Pd andCu.
 5. The semiconductor device according to claim 4, wherein the metalis Al.
 6. The semiconductor device according to claim 4, wherein themetal is an Al alloy containing the at least one alloying element. 7.The semiconductor device according to claim 6, wherein the content ofalloying element in the alloy ranges up to 5 wt %.